Subjective significance evaluation tool, brain activity based

ABSTRACT

A method and system for determining the subjective state of mind of a human subject presented with a test audible or visual stimulus. An electroencephalogram (EEG) recording unit is connected to the human subject, recording the subject&#39;s EEG when presented with one or more test stimuli. The recorded EEG signal is then transformed to a 3-D map in order to visualize the brain areas that were active when presenting the test stimuli. The given 3-D map of the test stimuli is then compared with reference 3-D maps of neutral and subjectively significant stimuli.

FIELD OF THE INVENTION

The present invention relates to a method and system for determining thesubjective state of mind of a human subject and in particular toassessment of neurophysiologic manifestations of subjectivesignificance.

BACKGROUND OF THE INVENTION

The neural substrates of emotional response have traditionally beenstudied using universal sets of emotionally loaded stimuli, regardlessof their subjective significance to the individual subject. Assessmentof the unique brain response to subjectively significant stimuli has notbeen studied before.

Deception is probably the best-studied mind states of all; most of thepresent lie-detection testing techniques use polygraph devices toexamine the peripheral autonomic response to relevant versus irrelevantquestions. Present day polygraph devices can combine measurements ofelectro-dermal skin conductance, blood pressure, respiration andperipheral vasomotor activity, minute changes in vocal response and facetemperature. The increase in autonomic response is interpreted as anattempt to deceive by the investigated subject. This basic principlebehind polygraph machine hasn't changed since its invention over 80years ago.

Lately, a newer method for lie detection which is based on examining theamplitude of the P300 component of event-related brain potentials wasproposed (Farwell, L. A. & Smith, S. S. J. Forensic Sci. 46, 135-143,2001). When a human subject is exposed to something that already isstored in memory, the brain emits an electrical response called a P300wave. The P300 is a non-specific brain response, elicited in response toinnovation, subject's own name, surprise and task related stimuli. Thisphenomenon occurs approximately 300 milliseconds after a meaningfulstimulus. The investigators extrapolate from an Electro-Encephalo-Graphy(EEG) recording clues to differentiate between relevant and irrelevantstimuli.

Another important known technique for mind state detection is thefunctional magnetic resonance imaging (fMRI). Images of the brain can betaken during activity at rest and during a specific behavior, thusdemonstrating the function of a particular area of the subject's brainas well as its structure. An extensive research is performed to detectthe part of the brain active during a lie. There are a number of reportsin the literature about success in detecting deception (Langleben, D. D.et al. NeuroImage 15, 727-732, 2002), (Kozel F. A. et al. BehavNeurosci. 2004 August; 118(4):852-6), (Ganis G et al. Cereb Cortex. 2003August; 13(8):830-6), a number of brain regions were found toparticipate in the deceptive response. This is an important discovery,since even though a subject might control his autonomic responses, thevery thought of deception will be detected. A few studies were also doneconcerning the subjective emotional experience of subjects. However,fMRI require a large and expensive and non-mobile instrumentation, andcannot get results within milliseconds resolution.

SUMMARY OF THE INVENTION

It is an object of the present invention to define brain activity tosubjectively significant stimuli.

Another object of the present invention is to characterize the timecourse of activity in the brain areas involved when exposed tosubjectively significant stimuli.

It is yet another object of the present invention to decide if a givenstimulus is subjectively significant or not to a human subject.

The present invention thus relates to a method for determining thesubjective state of mind of a human subject presented with a teststimulus and to a system for implementing said method. The methodcomprises the following steps:

-   (i) connecting an electroencephalogram (EEG) recording unit to the    human subject;-   (ii) presenting human subject with one or more test stimuli;-   (iii) recording the human subject's EEG;-   (iv) performing a 3-D reconstruction of the recorded EEG signal in    order to visualize the brain areas that were active when presenting    the test stimuli; and-   (v) comparing human subject's reactions to test stimuli to reference    data in order to determine if said one or more test stimuli are    subjectively significant or not.

The invention aims to determine if a given stimulus is subjectivelymeaningful to an individual or if it is neutral to that individual.

Preferably, several repetitions of the stimuli are used, so that eventrelated potentials (ERP) are recorded to enhance the signal to noiseratio. The test stimuli are compared with reference data that can beobtained from two sources: reactions to reference stimuli and/orpredetermined mapping of certain brain area known to be activated duringdifferent mind states.

In addition, the EEG signal is also recorded shortly before the stimulusis given in order to identify how the brain is active before beingexposed to the stimulus.

Measuring the reference stimuli is done in a similar way to measuringthe reactions to test stimuli. The human subject is presented with twosets of stimuli: a first set that comprises neutral, that isnon-subjectively significant stimuli for that individual; and a secondset that comprises subjectively significant stimuli for that individual.Measuring the reference stimuli is obtained using the following steps:(i) connecting an electroencephalogram (EEG) recording unit to the humansubject; (ii) preparing a list of significant and non significantreference stimuli; (iii) providing reference stimuli to the subject andrecording his ERP; (iv) using a computer unit to receive the recordedEEG and perform 3-D reconstruction of the EEG signal; and (v) performinganalysis of subject mental reaction to reference stimuli.

The test stimuli 3-D reconstruction is compared to the 3-Dreconstruction of neutral and subjectively significant stimuli in orderto assess if the test stimuli are subjectively significant or not.

The test stimuli, as well as the reference stimuli, can be providedthrough common audio-visual means such as earphones, a computer screen,a television monitor etc. The stimulus is controlled by a computer unitso that the time of delivering the stimulus is controlled with greatprecision. The EEG measures the responses in different brain areasmostly between 200 milliseconds (ms) and 950 ms after a stimulus isdelivered.

Optionally, Peripheral arterial tonometry (PAT) recordings can be usedto check peripheral autonomic response. PAT can be used instead of EEGreadings, though the results are less precise.

One or more human subjects are presented with a set of subjectivesignificant stimuli and a set on neutral stimuli. The EEG signalsrecorded are transformed to 3D maps of different brain parts active at agiven time point after the stimulus is received. The end result is twoset of maps of activities of different brain parts at a given time pointafter the stimulus is received. One set of maps corresponds to a neutralstimulus and the other set of maps corresponds to a subjectivelysignificant stimulus. Then if we wish to determine if a new stimulus issubjectively significant or not to a given individual, we can measurethe brain activity after receiving the stimulus and compare thereconstructed brain maps with the previous two sets of maps for neutralor subjectively significant stimuli. Comparing the new map to the tworeference maps (neutral and subjectively significant stimuli) will allowus to determine if the new stimulus is subjectively significant or notaccording to the resemblance of the new brains maps with either thebrain maps for neutral stimuli or the brain maps for subjectivelysignificant stimuli.

The invention may be used in a variety of applications including but notlimited to: a psychological evaluation tool, a tool for identificationof subjectively significant response in comatose patients, a tool foridentification of a specific response in demented patients, a tool forevaluation of psychological response in psychiatric patients andautistic children, a tool for evaluation of subjectively significantresponse in locked in syndrome patients, and as a lie detector.

Subjective significance evaluation tool, brain activity based, may alsoserve as a tool for evaluating residual specific brain activity inpatients with no motor response, as in comatose patients, locked insyndrome, or demented patients. It may also serve the evaluation ofspecific response in clinically depressed patients, or autisticchildren. In many psychiatric disorders, the therapy is difficult toachieve since the patient is not cooperative, and in many cases is noteven aware of the subconscious conflicts and drives which cause his/hersillness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows event related potentials (ERP) to subjectively significantand neutral names, grand average across 16 subjects, from 11 electrodes:Fp1, Fp2, F3, Fz, F4, C3, Cz, C4, P3, Pz, P4. Main ERP components aremarked: P1, N2, N400, P600. Note the enhanced components N2, N400 andP600 to subjectively significant stimuli.

FIG. 2 illustrates the time course of statistically significantactivation in the vicinity of 13 areas in each hemisphere tosubjectively significant and neutral names, indicated by time framesmarked in black. Note that responses to subjectively significant stimuliare: (1) overall enhanced compared to neutral names, more so in theleft-compared to the right hemisphere; (2) particularly enhanced in thevicinities of the middle and superior frontal gyri, Broca's area,Wernicke's area, insula, precentral gyrus, precuneus and posteriorcingulate gyrus in the left hemisphere; (3) prominent in the later timeframes in the vicinity of the middle, superior, and medial frontal gyri,as well as in the vicinity of the anterior, central and posteriorcingulate gyri.

FIG. 3 illustrates the time course and level of significant activationin the vicinity of specific brain areas in response to subjectivelysignificant vs. neutral stimuli. Level of activation in the vicinity ofeach area was computed as the percentage of voxels in the area that weresignificantly active at a given time frame, multiplied by their averaget value. The sections in the middle column of the figure show activationin the vicinity of a given area in response to subjectively significantstimuli, at the time of its peak activation, marked by an arrow on thegraph. The respective time courses of activation in the vicinity of eacharea, from 200 to 800 ms after stimulus onset, in response tosubjectively significant and to neutral stimuli, are also shown. Note:(1) Enhanced activation in response to subjectively significant stimuli;(2) differential pattern of activation in the vicinity of concurrentlyactive areas in response to subjectively significant compared to neutralnames; (3) prominence of the response in left hemisphere; (4) earlieronset of activation in the vicinity of the left middle frontal gyrus inresponse to subjectively significant stimuli.

FIG. 4 shows the main cortical sources contributing to the difference inERP components-results of a statistical comparison between subjectivelysignificant and neutral names during several time periods. For eachcomponent, the vicinity of the area found to be most involved in theresponse is shown. On the right, the corresponding time period fromwhich the sources were estimated is marked on the ERP waveform.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of various embodiments, referenceis made to the accompanying drawings that form a part thereof, and inwhich are shown by way of illustration specific embodiments in which theinvention may be practiced. It is understood that other embodiments maybe utilized and structural changes may be made without departing fromthe scope of the present invention.

Human behavior is affected by the subjective significance of events. Thepresent invention aims to define brain activity to subjectivelysignificant stimuli, and characterize the time course of activity in thebrain areas involved. Previous studies on the neural basis of emotionswere typically fMRI or PET studies (Crosson et al., 2002; Elliott etal., 2000, 2002; George et al., 1995; Gundel et al., 2003; Maddock etal., 2003) whose temporal resolution is in the order of seconds, whereasneural activity related to emotional processing and behavior is in thesub-second range. The present invention records Event-Related Potentials(ERPs) from the scalp and uses current density source estimation(Low-Resolution Electromagnetic Tomographic Analysis—LORETA;Pascual-Marqui et al., 1994; Pascual-Marqui, 1999; Pascual-Marqui R D,Michel C M, Lehmann D. Low resolution electromagnetic tomography: a newmethod for localizing electrical activity in the brain. InternationalJournal of Psychophysiology 1994, 18:49-65) to trace brain activity withtemporal resolution of milliseconds and spatial resolution in the orderof millimeters.

The neurophysiological basis of emotional response to stimuli has, forthe most part, been studied assuming the same affective valence for eachstimulus across all subjects (Bremner et al., 2001; Dietrich et al.,2001; Elliott et al., 2002; Hamann and Mao, 2002). Previous studies havedemonstrated the involvement of a neural network consisting of theprefrontal cortex, both temporal lobes and cingulate gyrus, in additionto other subcortical areas (amygdala, hippocampus), in the emotionalresponse to stimuli (Crosson et al., 2002; Devinsky et al., 1995;Elliott et al., 2000; Elliott et al., 2002; Esslen et al., 2004; Georgeet al., 1995; Imaizumi et al., 1997; Maddock and Buonocore, 1997;Maddock et al., 2003). Contradictory results were reported by studiesthat used such uniform sets of stimuli, regarding the involvement ofcertain areas (e.g. anterior cingulate gyrus, see Blair et al., 1999,and Esslen et al., 2004) and also regarding hemispheric lateralizationof the emotional response to stimuli (Jones and Fox, 1992; Davidson andIrwin, 1999; Esslen et al., 2004).

The invention uses subjective significance to the individual subject,rather than universal (objective) affective valence which is assumed tobe the same across all subjects. In general, subjectively significantstimuli are more prone to evoke behavior than other stimuli. When thesame set of stimuli, assumed to be universally affective, is used forall subjects, individual stimuli may have different affective valencefor different subjects. Thus, comparing brain activity to such stimuliacross subjects, for whom the same stimulus may carry a differentsubjective significance, does not allow isolating the net effect ofaffective valence.

For example, most verbal stimuli have more than one meaning and may haveseveral connotations; which complicates the determination of theirsubjective affective valence. In contrast, the subjective significanceof first names is mostly acquired from people familiar to the subjectthat carry the name. Therefore, first names may be suitable for studyingthe neurophysiological correlates of emotional significance.Furthermore, the very same first name may have an emotional value tosome subjects and be neutral to others, providing a better control forthe net effect of subjective emotional significance of the very samestimulus, than comparing different words. Several studies havedemonstrated the neural response to the subject's own name compared toneutral names (Berlad and Pratt, 1995; Perrin et al., 1999; Kampe etal., 2003). A comparison of the neural response to any subjectivelysignificant names with that to neutral names has not been reported.Moreover, previous studies describing the neural response to thesubject's own name did not trace the exact time course and order ofactivation of the brain areas involved.

The present invention identifies the specific neural response tosubjectively significant stimuli and traces neural correlates ofsubjective emotional response. The general brain response evoked by allsubjectively significant stimuli, with both negative and positivevalence is captured. Moreover, because some subjectively significantstimuli may have a mixed negative and positive connotation (e.g. thesubject's mother, depending on the context and personal experience),brain responses to subjectively significant stimuli are more readilydefined than to stimuli classified as ‘positive’ or ‘negative’.

Results of experiments conducted show a consistent and uniform patternof brain response to subjectively significant stimuli across allsubjects of the same group. In each of the subjects the response tosubjectively significant stimuli included neural activity in earliertime frames and in additional brain areas compared to the response toneutral stimuli. Activation up to 200 ms from stimulus onset wasapparently not uniform enough across subjects to be statisticallysignificant. During this early time period, several brain areas may havebeen differentially involved but did not attain significance in ouranalysis because of different time courses of activation to differentstimuli in different subjects. The general features that werestatistically significant across all subjects show that brain responsespecific to subjectively significant stimuli is characterized by 3features: (1) generalized enhancement of cortical activity; (2) aspecific localized response involving several distinct and concurrentlyactive brain areas at different times; (3) late (>700 ms) activityunique to subjectively significant stimuli, after the activity toneutral stimuli has subsided. The generalized enhancement of activation(see statistical results and FIGS. 2 and 3) may be part of an arousalresponse mediated by the amygdala and other parts of the limbic system(Critchley, 2003; Jones, 2003; Shekhar et al., 2003). Significantamygdalar activation was not found in our study, most probably becauseof its deep location within the brain and the fact that its neurons arenot spatially aligned.

According to one embodiment of the present invention, any audio and/orvisual stimulus can be tested if it is subjectively significant to anindividual, by creating 3-D reference maps of neutral and subjectivelysignificant stimuli delivered to other individuals from the same group.Defining the group of similar individuals depends on the context of whatis needed to be tested. If the stimulus tested is a word in a givenlanguage, as in the experiment described below, than all the individualsneed to be speakers of the same language. If the stimulus to be testedis a visual stimulus of a technical object, the group of referenceindividuals needs to be familiar with the technical object in order totest if it is subjectively significant. Other factors may also bedetermined in order to create a homogenous reference group, for example,age, health, right-handed or left handed people, sex, education etc.

According to another embodiment of the present invention, referencestimuli can be measured in a group of reference individuals that is nothomogeneous.

When measuring reference stimuli, subjectively significant stimuli canbe determined by various non-exclusive methods, for example, aquestionnaire, an interview or prior knowledge about the individual orsubject.

The present invention can be used in a variety of applications, forexample:

1. Truth Detection:

The present invention can serve as an important tool for legalinvestigations, including screening of court witnesses for intention tolie or a tool to probe the mental response of the prosecuted individualto crime evidences. Another use of the invention is in job screening andin other fields the polygraph device is currently used. Anotherapplication is in crime investigation whereas one can present suspectswith a number of probes that are unique details of a crime that only theculprit could know. A guilty subject is expected to have memory of theseprobes and therefore treat them differently than the innocent person.

2. Brain Computer Interface (BCI):

Current methods for using mental information to activate computers orrobots are based on plain EEG signals or on invasive devices. Althoughindividuals can learn to control and change at will certain aspects ofthe EEG signal, the current rate of information transfer with plain EEGrecording is very low. In one embodiment, the present invention enablesto perform online 3D reconstruction of the signal to capture the fullpotential of the data encoded in the EEG.

3. Improvement of Psychotherapeutical Treatment.

The psychiatric staff meets many problems diagnosing and treating thesesubjects, since the behavior response is not a good indicationconcerning the success/failure of the therapy. The present invention canserve in psychological and psychiatric investigation, in order to findwhat is really important to the patient, or what is the source of theproblem, in order to build a therapeutic plan and find out what shouldbe the focus of the therapy. A mental response reader which is able toreveal specific response to therapeutic inquiries can greatly assist inthe treatment of a range of mental conditions such as autistic,psychotic and depressed patients and patients with post traumatic stressdisorder (PTSD). In addition, especially in PTSD patients the presentinvention enables identifying stimuli which are related to the trauma,which will be efficient to expose the patient to, or to avoid.

4. In ‘locked in’ and comatose patients, in whom the level of corticalactivity can be probed and specific brain activity to significantstimuli will be assessed (the technique of the invention also allows forquick testing whether the comatose patient is able to hear or to see).

5. Revealing information (and subjective significance of objects) inpatients who are unable to talk—in autism, locked in syndrome, comatosepatients, severely disabled patients or patients in a catatoniccondition; and in patients who are unwilling to talk (a lie detector).

Experiment Conducted

1. Methods

1.1 Subjects

Sixteen right handed, native Hebrew speaking, healthy normal volunteers(7 males and 9 females; mean age: 22.7, SDZ2.8 years) with no hearingcomplaints nor evidence of neurological disorders participated in thisstudy. Since subjects' first names were used as auditory stimuli in theexperiment, subjects were chosen so that their first names were 2syllable common Hebrew names, which began with a sonorant. Subjects werepaid for their participation. Experimental procedures were approved bythe Institutional Review Board for experiments involving human subjects(Helsinki Committee).

1.2 Experimental Procedure

Potentials were recorded from tin electrodes placed according to the10-20 system in an electrode cap (Electro-Cap International Inc., Eaton,Ohio, USA). Activity was recorded (Ceegraph IV Biologic Systems Corp.IL, USA) from 19 locations: Fp1, Fp2, F7, F3, Fz, F4, F8, T3, C3, Cz,C4, T4, T5, P3, Pz, P4, T6, O1, O2. Three additional tin electrodes,external to the cap, were attached to the right and the left mastoids(A1 and A2), and a third below the left eye, referenced to Fz, tomonitor eye movements (EOG). In total, EEG was recorded from a21-electrode array, in addition to an EOG channel. A ground electrodewas placed on the left forearm. All EEG electrodes were referenced to anelectrode midline on the chin, far from the brain, from facial andlingual muscles and in the midline, thus avoiding asymmetry distortions.Impedance at each electrode was maintained below 5 kΩ.

Subjects listened to names presented binaurally by earphones andperformed an identification task, pressing a button in response to eachof 3 target names ending with a pre-assigned consonant, from arepertoire of 29 names. Subjects performed the task reclining in anadjustable armchair in an acoustically isolated chamber. Subjects wereinstructed to avoid eye movements and blinking as much as possible, andto keep their gaze on a fixed point in front of them during taskperformance.

1.3 Stimuli

All stimuli were two syllable (700 ms duration) common Hebrew firstnames, beginning with a sonorant so that all stimuli had similaracoustic onset properties. In each experiment, 29 names were randomlyrepeated. The stimuli were identical for each group of 4 subjects, andincluded the subjects' own names, and 5 names of important persons ineach subject's life (a present or former spouse, close family members,close friends, and a hated person). Any given name in the list was thussubjectively significant to at least one member of the group and wasneutral to others. The stimuli also included 3 target names that endedwith the same consonant. Target names were neutral to all subjects. Dataabout names of important persons in the subject's life were collectedduring a short interview 1-2 months before the experiment. Finalassessment of the subjective significance and affective valence of eachof the names in the list was conducted by an interview after theexperiment.

Stimuli were recorded from a native Hebrew speaker and saved asdigitized sound files. Stimuli were presented binaurally with aninter-stimulus interval (between the onset of a name and onset of thefollowing name) of 2.2 s.

1.4 Experimental Paradigm

Subjects were instructed to press a button when they heard a name thatends with a certain consonant (target name). Subjects received 10blocks, each consisting of 7 randomly distributed repetitions of each ofthe 29 names in the list. The list of names included in the experimentwas identical for each group of 4 subjects. Short breaks were takenafter each block.

1.5 Interview

Subjective affective significance of each name was assessed after theexperiment in an interview, based on a structured questionnaire whichwas validated both statistically and by using an autonomic activitymeasure (peripheral arterial tonometry). The subject was asked abouteach name separately. The questionnaire included 46 dichotomous andrating questions for each person bearing each name. A subjectivesignificance score was computed accordingly for each name, for eachsubject. Names with scores higher than a standard deviation above thesubject's own score averaged across all names were consideredsubjectively significant to that subject, and names with scores lowerthan a standard deviation below the subject's own averaged score wereconsidered neutral.

For each subject, names were ranked according to their individualaffective significance. Thus, a given name could be significant to onesubject and neutral to another, and the net effect of subjectivesignificance on brain activity could be isolated.

1.6 ERP Recording and Measurement

Potentials from the EEG (X100,000) and EOG (X20,000) channels wereamplified, filtered (0.1-100 Hz, 6 dB/octave slopes), digitized with a12 bit A/D converter at a rate of 256 samples/s, and stored forsubsequent off-line analysis.

Continuous records were segmented beginning 300 ms pre-stimulus until2700 ms after stimulus onset (3 s total analysis time), and averagedafter eye-movement correction based on eye-channel/EEG correlations(Attias et al., 1993).

Potentials were averaged separately for each non-target first name, foreach subject. Potentials to target names were not included in theanalysis. Thus, all the stimuli whose responses were analyzed, neutraland subjectively significant, were non-targets in this task. Twoseparate averages were derived for each subject: (1) for the 3 mostsubjectively significant names; (2) for the 3 least subjectivelysignificant (‘most neutral’) names, according to the interview results(see above). In this report, ERPs to all subjectively significant nameswere averaged together, irrespective of positive or negative valence.The effects of valence will be reported in a separate report. Theaveraged ERPs were low-pass filtered with a cutoff at 20 Hz.

1.7 Current Density Estimations and Statistical Analysis

Neural sources of scalp-recorded potentials were estimated using theLORETA procedures for estimating source current density distribution ina 3D Talairach space of the brain's gray matter. The LORETA procedurecomputes current density under the assumption that for each voxel thecurrent density should be as close as possible to the average currentdensity of the neighboring voxels (‘smoothness assumption’). LORETA hasbeen shown to have a 7 mm spatial resolution, and found superior toother localization methods in localizing deep (subcortical) sources(Pascual-Marqui et al., 1994; Pascual-Marqui, 1999; Pascual-Marqui etal., 2002).

LORETA current density estimations were performed on each subject's ERPwaveforms, separately for subjectively significant and for neutralnames. In order to negate non-specific effects related to readiness andmotor preparation, a baseline level and distribution of activity wasdefined as the average for each voxel over the 100 ms preceding stimulusonset. Only activity significantly different than this baseline activitywas analyzed. Significance of activity was determined by nonparametric(SnPM) paired comparisons of time frame by time frame current density ineach voxel with baseline, in the 800 ms following name onset. The SnPMmethod estimates the probability distribution by using a randomizationprocedure; it corrects for multiple comparisons, and has the highestpossible statistical power (Nichols and Holmes, 2002). Specifically, inour study we used the ‘pseudo-t’ statistic which reduced noise in thedata by averaging over adjacent voxels (Nichols and Holmes, 2002).Comparisons were conducted separately for subjectively significant andfor neutral names, compared to baseline. An additional comparison (SnPM)was conducted for the responses to subjectively significant compared toneutral names. Level of activation in each area was computed as thepercentage of voxels in the vicinity of a specific area that weresignificantly active at a given time frame, multiplied by their averaget value. Analysis of Variance procedures were conducted in order toassess the effects of subjective significance (significant vs. neutral),laterality (left vs. right hemisphere) and their interaction on brainactivity. Probabilities below 0.05, after Geisser-Greenhaus andBonferroni corrections (when appropriate), were considered significant.

2. Results

FIG. 1 presents the potentials recorded from 11 of the 21 channels, inresponse to subjectively significant and to neutral names. Overall,subjectively significant stimuli were associated with enhanced activityrelative to neutral stimuli. Table 1 summarizes activity in the vicinityof areas found to be significantly active above baseline (P<0.05) inresponse to subjectively significant and to neutral names duringdifferent time periods after stimulus onset. The main areas specificallyinvolved in response to subjectively significant stimuli were in thevicinities of Wernicke's area and Wernicke's homologous area in theright hemisphere, of Broca's area, of left middle, medial and superiorfrontal gyri (at different times), of the right auditory cortex, of theleft hippocampus and of the left precuneus. TABLE 1 Areas involved inthe response to subjectively significant and neutral stimuli Areasinvolved in response to both Areas involved only in response to neutraland subjectively significant subjectively significant stimuli stimuliLeft Right Left Right Latency (ms) hemisphere hemisphere hemispherehemisphere 200-350 Central cingulate Mid. temporal G. Wernicke's G. Sup.temporal G. homologuearea Insula 350-500 Ant. cingulate G. Ant.cingulate G. Sup. frontal G. Orbital G. Central cingulate Centralcingulate G. Mid. frontal G. Mid. temporal G. Inf. frontal G.Supramarginal G. Inf. Frontal G. Med. frontal G. G. Inf. temporal Med.frontal G. Rectal G. Postcentral G. G. Rectal G. Sup. temporal G.Wernicke's Subcallosal G. Insula homologue Inf. parietal Precentral G.area lobule Parahippocampal G. Wernicke's area Subcallosal G. Broca'sarea Uncus Insula Sup. temporal G. Precentral G. Ant. cingulate G.500-600 Ant. cingulate G. Ant. cingulate G. Wernicke's area Mid.temporal Central cingulate Central cingulate G. Broca's area G. G. Inf.frontal G. Sup. temporal Med. frontal G. Med. frontal G. G. Mid. frontalG. Sup. frontal G. Sup. frontal G. Rectal G. Rectal G. Subcallosal G.Subcallosal G. Uncus Parahippocampal Parahippocampal G. G. HippocampusInsula Sup. temporal G. Uncus Precentral G. 600-700 Ant. cingulate G.Ant. cingulate G Wernicke's area Insula Central cingulate Centralcingulate G. Broca's area Precuneus G.. Inf. frontal G. Mid. frontal G.Med. frontal G. Med. frontal G. Precentral G. Inf. Frontal G. Mid.frontal G. Sup. temporal G. Sup. Frontal G. Sup. frontal G. Inf.temporal G. Insula Rectal G. Inf. parietal lobule Sup. paritetal lobulePrecuneus Postcentral G. Hippocampus 700-800 Ant. cingulate G. Ant.cingulate G. Broca's area Sup. frontal G. Central cingulate Centralcingulate G. Mid. frontal G. Mid. frontal G. G. Inf. frontal G. Inf.frontal G. Sup. frontal G. Med. frontal G. Precentral G. Med. frontal G.Rectal G. Sup. temporal G. Rectal G. Orbital G. Insula PrecuneusAbbreviations used:G, gyrus;Ant., anterior;Mid, middle;Sup, superior;Inf, inferior;Med, medial.

FIG. 2 presents bar graphs of the timing of activation in the vicinityof the main areas involved in the left and right hemispheres,demonstrating the more prevalent activation of left hemispherestructures in response to subjectively significant stimuli. Note theconcurrent activation of the brain areas involved. FIG. 3 shows thelevel of brain activity in the vicinity of several areas in response tosubjectively significant-compared to neutral stimuli. The graphs alsoshow the comparable activation of homologous regions in bothhemispheres. The areas detailed in the figures were the mostsignificantly activated brain regions in the time period between 200 and800 ms after stimulus onset. An overall enhanced brain activity wasfound in response to subjectively significant compared to neutralstimuli [F(2,52)=12.39; P<0.005]. Enhanced activation was found inresponse to both subjectively significant and neutral stimuli in theleft hemisphere compared to the right hemisphere [F(2,52)=12.55;P<0.005]. A significant interaction between subjective significance andlaterality was found [F(2,52)=9.45; P<0.01], with the subjectivesignificance effect significantly more prominent in the left hemisphere.FIG. 4 summarizes activity in the vicinity of the main areas found to bespecifically involved in the response to subjectively significantstimuli, when their activity was statistically compared to that ofneutral stimuli, during peaks of activation in the ERP waveforms.

3. The Questionnaire

The questionnaire used in the experiment (see Appendix) intended tomeasure subjective significance to the subject of first names.Subjective significance (or subjective affective valence) of first namesto the participant was defined as subjective significance of people inthe participants' life that bear that name. Verbal stimuli in generalhave multiple dimensions (e.g., meaning, grammatical function, affectivevalence). In contrast, first names have only one major semanticdimension (subjective valence). The subjective meaning of a first nameis mostly acquired from familiar people in the participants' life thatbear this name. The questionnaire was developed in order to assesssubjective significance of people in the participant's life.

The questionnaire was planned to target 3 dimensions of subjectivesignificance: (1) General emotional significance; (2), Negative impact;and (3) recency of contact (closeness, past and present). Specific itemswere included to assess each of these factors. Questionnaire items wereformulated to assess the following affective aspects: closeness, anger,liking, dislike, frustration, love, hatred. Additional items wereincluded to assess duration and frequency of relationship. In addition,items targeted past traumatic experiences.

Although the questionnaire was originally developed to assess subjectivesignificance of stimuli in the context of an event related brainpotentials study, it can also serve other psychological and socialstudies of relationship and personality, beyond the realm of brainresearch. This questionnaire may also serve clinical practice, inmapping complicated relationships in the participant's life, assessingsupporting relationships and characterizing relationships related totraumatic events in the participant's life.

The questionnaire, in its final form, included 46 yes\no and ratingquestions (see Appendix) on each name. The original questionnaireincluded 70 questions, and was reduced after the factor analysis. Eachparticipant was questioned about all names in a list of 30 two syllable,common Hebrew names. Initially, the questionnaire included 70 items thatwere directed at 3 factors: (1) general emotional significance; (2)negative impact; and (3) recency of contact. Items were first formulatedso that every factor that might affect the relationship between 2 peoplewould be included and probed, with some redundant items asking about thesame issues in different manners.

Emotional content questionnaires were addressed as well. Later, after afactor analysis, the detailed questionnaire was reduced to include onlythe most relevant and discriminating items. In rating answers, apositive answer indicated an emotionally loaded name, and the numericalvalue reflected magnitude.

Validation of the affective valence score was performed using aplethysmographic measure for sympathetic activation. Sympatheticactivation is known to be affected by emotional response. Theplethismographic measurement was conducted on the same day as theinterview, using a peripheral arterial tone (PAT) device. The PAT device[Itamar Medical; Caesarea, Israel] is a portable device based on the PATsignal. Finger pulse wave was measured by a plethysmographic technique(Bar, A.; Pillar, G.; Dvir, I.; Sheffy, J.; Schnall, R. P.; Lavie, P.Evaluation of a portable device based on peripheral arterial tone forunattended home sleep studies. Chest. 2003, 123(3):695-703; Dvir, I.;Adler, Y.; Freimark, D.; Lavie, P. Evidence for fractal correlationproperties in variations of peripheral arterial tone during REM sleep.Am. J. Physiol. Heart. Circul. Physiol. 2002, 283(1):H434-9). The PATsignal was recorded while the participants were reclining in acomfortable armchair in an acoustically isolated chamber, listening tonames (all 30 names in the list) presented by earphones. Theparticipants were instructed to listen to the names and think aboutpersons they knew bearing those names, without pressing any button. Theinterval between names was 20 seconds, to accommodate the slow timecourse and the known latency of the autonomic response (severalseconds). Mean peak amplitude (MPA) was computed for 5 intervals of 3seconds after each stimulus, using a baseline of 3 seconds before eachstimulus onset, in order to track the minimal amplitude of PAT followingthe stimulus. The interval with the minimal MPA after the stimulus waschosen. PAT response was determined as the % of PAT signal change:[(minimum MPA after the stimulus—MPA at baseline)/MPA at baseline]×100.Those computations were conducted for the 3 most subjectivelysignificant and 3 least subjectively significant names for eachparticipant. ANOVA was performed to assess the effect of affectivevalence on the PAT response.

Informed consent was obtained. The questionnaire was filled in an oralinterview. Each participant was asked the entire questionnaire abouteach person he knew bearing a name from the list of 30 common Hebrewnames. For each name, the following procedure was repeated: Participantwas asked to recall all the persons he knew, in the present and in thepast, who bear that specific name. Then, participants were asked tospecify their relationship with each person mentioned for that name(fellow student, a childhood friend, a family member, the bus driver,etc.). Participants were subsequently asked to shortly answer 70questions about that person. Specific questions were included if theparticipant was, or was not in contact with that person at the time ofinterview. The questions were either dichotomic yes/no questions, orrating questions (rating between 1 and 5). In the rating questions, ‘1’meant “not at all”, and ‘5’ indicated “applies very much”. Answers weremarked by the interviewer. Items were scored so that higher scores meanhigher subjective significance.

Rating values were transformed to dichotomous values by computing theaverage response for each participant on a given item across allquestionnaires (across different persons' names) completed by thatparticipant. Responses above that average were assigned a positivevalue, and those under-negative. In all, 455 questionnaires werecompleted and processed. Thus, most subjects rated more than one personfor each name—in total 30-40 questionnaires were filled by each subject.

Factor analysis was conducted and internal consistency (for each factor)was measured. Internal consistency was assessed within each factor,between all the questions in this factor. Redundant, ambiguous, or lowitem-to-total correlation items were removed from the questionnaire andsubsequently 46 of the original 70 items were included in the finalVarimax rotation factor analysis.

Following the initial analysis, 46 of the original 70 items were founduseful and were included in the questionnaire. The final items arepresented in the Appendix.

Factor analysis demonstrated that a 3-factor solution best explained thevariance in the responses to the questionnaire (98%). Factor 1(“subjective significance”; 26 items) explained 54% of variance withloadings ranging from 0.76 to 0.43. Factor 2 (“negative connection”; 12items) contributed an additional 25% of the explained variance withloadings ranging from 0.72 to 0.41, and factor 3 (“recency of contact”;8 items) contributed 19% of the additional variance, with loadingsranging from 0.76 to 0.44. Table 1 lists the factor loadings for eachitem and Table 2 details the questions that were associated with eachfactor. When data were randomly halved, and factor analysis conductedseparately for the two halves, the factor structure results wererepeated.

All factors demonstrated good reliability (see Table 3). Factor 1 had aCronbach's alpha of 0.94, while factor 2 was associated with an alpha of0.88 and factor 3—0.82.

Validity of the subjective significance questionnaire was tested usingan autonomic activation measure—the PAT signal. Correlation between thesubjective significance score and the autonomic response measure wassignificant (p<0.05).

Enhanced autonomic response (reduced peak amplitude) was found aftersubjectively significant stimuli compared to neutral stimuli[F(2,8)=10.12, p<0.05]. Thus, subjective significance correlated wellwith high % change of the PAT signal.

APPENDIX

Following are the final 46 items of the questionnaire and their originalnumberings:

Factor 1:

Yes\No questions

1. Would you prefer to see that person more frequently than you do atthe present?

2. Do you see each other on your initiative more than once a week?

6. Has that person ever been the closest person (or the second closest)to you?

7. Has your acquaintance with that person affected your life?

9. Was your acquaintance with that person significant for you?

12. Is your acquaintance with that person still affecting you today?

13. Have you experienced significant events with that person?

15. Was that person present in any significant event you haveexperienced?

18. Has that person ever appeared in your dreams?

19. Have you ever felt closeness to that person?

Rating questions (on a scale of 1-5).

3. How much is this person in your life (Physically or not)? [Verymuch=5; not at all=1].

4. Rate the level of your closeness with that person. [Very much=5; notat all=1].

5. Rate the maximal level of closeness you have ever had with thatperson in the past [Very much=5; not at all=1].

8. Rate how much your acquaintance with that person affected your life[Very much=5; not at all=1].

10. Rate how significant your acquaintance with that person was to you[Very significant=5; not at all=1].

11. Rate how significant this person was to you at the time he was mostsignificant for you. [Very significant=5; not at all=1].

14. Rate how intense the most intense experiences you have everexperienced with that person were. [Very intense=5; not at all=1].

17. Rate how often you find yourself thinking about this person. [Veryoften=5; not at all=1].

20. How often does this person come to your mind? [3 or more times aday=5; never=1]

21. How often do you think about this person? [All the time=5; never=1].

22. How significant is this person to you? [Very much=5; not at all=1].

23. How much has this person affected your life? [Very much=5; not atall=1].

24. How much do you like this person? [Very much=5; not at all=0].

25. What is the duration of your acquaintance? [All of my life=5; justone coincidental meeting=1].

26. How often do you see each other on your initiative? [Every day=5;never=1].

Factor 2:

Yes\No questions

27. Would you prefer if you never met that person?

28. Has that person ever frustrated you?

30. Do you remember having any fights or quarrels with that person?

31. Has that person ever hurt you?

33. Have you ever hurt that person?

35. Have you ever hated that person?

36. Were you ever angry at that person?

Rating questions (on a scale of 1-5).

29. Rate how much frustration this person has ever caused you. [Verymuch=5; not at all=1].

32. Rate how much you were hurt by that person. [Very much=5; not atall=1].

34. Rate how much you have hurt that person. [Very much=5; not atall=1].

37. How often does this person make you nervous? [Very often=5; not atall=1].

38. How much do you dislike this person? [Very much=5; not at all=0].

39. In case you are not in contact with that person right now: rate howsignificant your acquaintance was at the time you were in contact. [Verysignificant=5; not at all=1].

Factor 3:

Yes/No questions

40. Did the relationship end abruptly?

In case the participant doesn't see that person any more:

41. Has the relationship ended in a way you were not satisfied with, orin a traumatic way?

42. Do you remember your last meeting?

45. Has the relationship ended during the last 5 years?

46. Would you like to renew contacts with that person?

Rating questions (on a scale of 1-5).

43. In case you are not in contact right now—rate how intense your lastmeeting with that person was (if you can remember it). [Very intense=5;not at all=1].

44. In case you are not in contact right now—rate the intensity of yourseparation [very painful, I could not avoid thinking about this personall the time=5; I haven't even noticed the separation=1].

Although the invention has been described in detail, neverthelesschanges and modifications, which do not depart from the teachings of thepresent invention, will be evident to those skilled in the art. Suchchanges and modifications are deemed to come within the purview of thepresent invention and the appended claims.

1. A method for determining the subjective state of mind of a humansubject: presented with a test stimulus comprising the steps of: (i)connecting an electroencephalogram (EEG) recording unit to the humansubject; (ii) presenting human subject with one or more test stimuli;(iii) recording the human subject's EEG; (iv) performing a 3-Dreconstruction of the recorded EEG signal in order to visualize thebrain areas that were active when presenting said one or more teststimuli; and (v) comparing human subject's reactions to test stimuli toreference data in order to determine if said one or more test stimuliare subjectively significant or not.
 2. A method according to claim 1,wherein said reference data comprises 3-D reconstructions of EEG signalsof subjectively significant stimuli and/or 3-D reconstructions of EEGsignals of neutral stimuli.
 3. A method according to claim 2, whereinsaid 3-D reconstructions of EEG signals of neutral stimuli is obtainedfrom reactions to neutral stimuli and/or from predetermined mapping ofcertain brain area known to be activated during different mind states.4. A method according to claim 2, wherein said 3-D reconstructions ofEEG signals of neutral stimuli are obtained using the following steps:(i) connecting an electroencephalogram (EEG) recording unit to the humansubject; (ii) preparing a list of significant and neutral referencestimuli; (iii) providing reference stimuli to the subject and recordinghis ERP; (iv) using a computer unit to receive the recorded EEG andperform 3-D reconstruction of the EEG signal; and (v) performinganalysis of subject mental reaction to reference stimuli.
 5. A methodaccording to claim 1, wherein said test stimuli are provided viaearphones, a computer screen and/or television monitor.
 6. A methodaccording to claim 1, further including the step of recording the humansubject's Peripheral Arterial Tonometry (PAT).
 7. A method according toclaim 1, for use in one or more of the following applications: apsychological evaluation tool; a tool for identification of subjectivelysignificant response in comatose patients; a tool for identification ofa specific response in demented patients; a tool for evaluation ofpsychological response in psychiatric patients; a tool for evaluation ofpsychological response in autistic children; a tool for evaluation ofsubjectively significant response in locked in syndrome patients; or alie detector.
 8. A method according to claim 1, wherein neural sourcesare detected.
 9. A method according to claim 1, wherein comparing humansubject's reactions to test stimuli to reference data reveals specificbrain responses.
 10. A system for determining the subjective state ofmind of a human subject presented with a test stimulus comprising: (i)an electroencephalogram (EEG) recording unit connected to the humansubject; (ii) means for providing stimuli to the human subject; (iii) acomputer unit adapted for receiving the recorded EEG signal andperforming a 3-D reconstruction of the recorded EEG signal in order tovisualize the brain areas that were active when presenting said teststimuli; and (iv) means for comparing human subject's reactions to teststimuli to reference data.
 11. A system according to claim 10, whereinsaid reference data comprises 3-D reconstructions of EEG signals ofsubjectively significant stimuli and/or 3-D reconstructions of EEGsignals of neutral stimuli.
 12. A system according to claim 11, whereinsaid 3-D reconstructions of EEG signals of neutral stimuli is obtainedfrom reactions to neutral stimuli and/or from predetermined mapping ofcertain brain area known to be activated during different mind states.13. A system according to claim 11, wherein said 3-D reconstructions ofEEG signals of neutral stimuli comprises: (i) an electroencephalogram(EEG) recording unit connected to the human subject; (ii) a list ofsignificant and neutral reference stimuli; (iii) means for providingreference stimuli to the subject and recording his ERP; (iv) a computerunit adapted to receive the recorded EEG and perform 3-D reconstructionof the EEG signal; and (v) means for performing analysis of subjectmental reaction to reference stimuli.
 14. A system according to claim10, wherein said test stimuli are provided via earphones, a computerscreen and/or television monitor.
 15. A system according to claim 10,further comprising means for recording the human subject's PeripheralArterial Tonometry (PAT).
 16. A system according to claim 10, for use inone or more of the following applications: a psychological evaluationtool; a tool for identification of subjectively significant response incomatose patients; a tool for identification of a specific response indemented patients; a tool for evaluation of psychological response inpsychiatric patients; a tool for evaluation of psychological response inautistic children; a tool for evaluation of subjectively significantresponse in locked in syndrome patients; or a lie detector.
 17. A systemaccording to claim 10, wherein neural sources are detected.
 18. A systemaccording to claim 10, wherein comparing human subject's reactions totest stimuli to reference data reveals specific brain responses.